High octane will not harm your engine in any way. The octane slows the burn, preventing pre ignition, or knocking in high compression, or big bore engines. For the sake of power and cost of fuel, use the lowest octane that will not cause your engine to knock. Switch to low octane gas, you should notice a slight increase in power and fuel milage.
(P.S. I'll have my degree in Petroleum Engineering Dec 2006)